Hydroxy-functional acrylic resins of relatively low molecular weight (M.sub.n =about 1000 to 5000) are valuable reactive intermediates for making high-performance coatings and other thermoset polymers. The resins are crosslinked with melamines, polyisocyanates, epoxies, and other crosslinkers to give useful thermosets.
We recently described new hydroxy-functional acrylic resins (see U.S. Pat. Nos. 5,475,073, 5,525,693, and 5,571,884) that incorporate recurring units from allylic alcohols or alkoxylated allylic alcohols and ordinary acrylate monomers. The resins are useful for many common thermoset polymers (see U.S. Pat. Nos. 5,534,598 and 5,480,943). Unlike acrylic resins previously known, preparation of these resins does not require a reaction solvent or chain-transfer agent to control reactivity and molecular weight. The resins are less costly because allyl alcohol and ordinary acrylate monomers are used instead of hydroxyalkylacrylates. In addition, the resins have exceptionally low viscosities, which makes them valuable for high-solids, low-VOC formulations.
Despite the cost and formulating advantages of hydroxy-functional acrylic resins derived from allyl alcohol and other allylic monomers, the resins have some drawbacks. First, the advantage of using an allyl monomer, which helps to regulate molecular weight and reactivity, is offset somewhat by the difficulty in getting all of the allyl monomer to react completely within a reasonable time period. Usually, an excess of the allyl monomer is kept in the reaction mixture throughout the polymerization, and excess unreacted allyl monomer is removed by vacuum stripping and/or stripping with water, steam, or inert gas. The examples of U.S. Pat. No. 5,475,073 illustrate the need for vacuum stripping to remove unreacted allyl monomers. Unfortunately, vacuum stripping adds to cycle time and multiplies utility costs. Modification of commercial reactors to accommodate vacuum stripping is often costly and impractical. In addition, the stripped allyl monomer must be recycled and reused to make the process affordable.
The need to vacuum strip allyl monomers also limits the kinds of allyl monomers that can be used and the types of resins available. For example, U.S. Pat. No. 5,475,073 teaches that propoxylated allyl alcohols having an average of only 1 or 2 oxypropylene units can be used because higher alkoxylated allylic alcohols cannot be easily stripped from the polymer. Preferably, alkoxylated allyl alcohols having more than 2 oxyalkylene units could be included because a wider variety of hydroxy-functional acrylic resins could be made.
Another key drawback relates to resins made by the earlier process that derive from allyl monomers containing mostly secondary hydroxyl groups, e.g., propoxylated allyl alcohol-based resins. These resins can be slow in curing, especially in applications (such as automotive refinishing) for which room-temperature curing is preferred.
In sum, an improved process for making hydroxy-functional acrylic resins is needed. Preferably, the process would retain the benefits of the earlier allyl monomer-based acrylic resin process (U.S. Pat. No. 5,571,884): low raw material cost, polymerization without solvents or chain-transfer agents, low-viscosity products. In addition, however, a valuable process would give resins that cure rapidly even at room temperature. Ideally, the process would eliminate the need to vacuum strip and recycle allyl monomers, and would permit the use of higher alkoxylated allylic alcohols.